PDCD1 and neoplasm: Tumor-infiltrating lymphocytes (TILs) express a variety of immune checkpoints, including programmed cell death protein-1 (PD-1), cytotoxic T-lymphocyte antigen-4 (CTLA-4), T-cell immunoglobulin, and mucin-domain containing-3 (TIM-3), lymphocyte activation gene-3 (LAG-3), and T-cell immunoglobulin and ITIM domain (TIGIT) (19–21).